This invention relates to a method of producing piezoelectric oxide material and more particularly to a method of manufacturing a single crystal of lithium tantalate.
A single crystal for optical purposes has a piezoelectric property and is used with, for example, an elastic surface wave device. The single crystal of lithium tantalate is produced by various single crystal-growing methods, such as a pulling-up method, for example, a Czochralski method, pulling down method and laterally pulling method. The single crystal of lithium tantalate having a melting point of about 1,650.degree. C presented difficulties in being manufactured in a platinum crucible. Hitherto, therefore, the single crystal of lithium tantalate has been produced in a crucible prepared from iridium having a high melting point in an atmosphere of reducing or inert gas. However, a single crystal of lithium tantalate produced by the above-mentioned prior art process had many crystal defects including oxygen defect. These defects could not be eliminated even by a subsequent heat treatment.
Such crystal defects led to the prominent dislocation density of the single crystal of lithium tantalate (said density being of the order of 10.sup.5 to 10.sup.6 lines/cm.sup.2 ; see Journal of Crystal Growth, 24-25, pp 432 to 436, 1974), and in consequence the low mechanical strength thereof, thus presenting difficulties in being applied, for example, as a substrate of an elastic surface wave device.
Further, the single crystal of lithium tantalate produced in a crucible prepared from very expensive iridium unavoidably became costly. Moreover, when about ten single crystals of lithium tantalate were manufactured, the iridium crucible decreased in wall thickness or was bored with small holes, requiring repair now and then what was worse, repair was considerably expensive due to the high cost and high melting point of iridium.
The conventional method of manufacturing a single crystal of lithium tantalate in an iridium crucible had the drawbacks that the product was expensive and indicated a noticeable dislocation density, proving unadapted to be used as a surface wave intermediate frequency filter of, for example, a color television receiving set.
On the other hand, production of a single crystal of lithium niobate in a crucible prepared from a platinum-rhodium alloy is set forth in the research report No. 3027 by Yoichi Niizeki et al bearing the title "Growth and properties of a single crystal of lithium niobate" (issued on Oct. 20, 1966 by the Electric Telecommunication Institute of Nippon Telegraph and Telephone Public Corp.) on page 10 of this literature, the following passage appears: "rhodium was so readily soluble in melt of lithium niobate (LiNbO.sub.3) that a crucible of platinum-rhodium alloy used in view of its anticipated prominent resistance to high temperature turned porous and proved unusable, though the melt of lithium niobate was kept at a temperature 500.degree. C lower than the melting point of the platinum-rhodium alloy". Therefore, it has been considered impossible to use a crucible prepared from a platinum-rhodium alloy is manufacturing a single crystal of lithium tantalate having a similar crystalline structure to that of a single crystal of lithium niobate.